As is well-known, silicon is an essential alloying element in the production of cast iron, particularly the group of cast irons known as gray irons. Silicon not only promotes graphitization of carbon, but also increases the hardness and strength of the ferrite phase. Common commercially used gray irons include approximately 2.25 weight percent silicon. Depending upon the required properties, however, the silicon content may vary from 2.00 to 3.00 weight percent.
Heretofore, almost all of the silicon has been supplied by the initial cupola charge. In other words, the silicon contained in the metallics and in auxiliary additives in the cupola charge provide the silicon necessary to meet the specifications of the melt. In determining the composition of the initial charge, consideration must be given to the fact that silicon is lost through oxidation during melting. Silicon loss is particularly acute with a basic or neutral slag. Basic or neutral slags are desirable, however, because they favor a high carbon curve.
The amount of silicon lost ranges between 10% and as much as 40% depending on the cupola and the particular operating conditions. Hence, excess silicon must be provided in the initial charge to insure that the desired amount of silicon will be present in the melt. Even though excess silicon is provided in the initial charge, care must nevertheless be taken during melting to avoid a loss of silicon in excess of the predicted loss. Obviously, if the silicon loss during melting exceeds the predicted loss, the silicon content of the melt will fall below the desired level even though excess silicon was proved in the initial charge.
Due to the fact that the initial charge must have a relatively high silicon content (on the order of 2.6 - 2.75% to produce 2.25% at the cupula spout) it has long been felt that a large portion of the ferrous-base material, or metallics, in the charge must have relatively high silicon contents. Hence, ferrous-base materials, such as pig iron, foundry returns and purchased cast iron scrap, are used in greater quantities than steel scrap because the former have significantly higher silicon contents than the latter. This adds, however, to the material cost of the initial charge since steel scrap is less expensive than the average cost (or value) of pig iron, foundry returns, and purchased cast iron scrap. In addition, silicon must also be added to the charge in the form of auxiliary additives to give the desired amount. Silicon additions are generally made in the following forms: (1) silicon carbide, briquettes which contain approximately 38 - 75% silicon carbide, the compound silicon carbide containing 70% silicon and 30% carbon, (2) ferrosilicon which contains approximately 50%, 75% or 85% silicon depending on the type used, (3) silvery pig which contains 8 - 20% silicon and (4) silicon briquettes. One or more of these auxiliary silicon additives are used to increase the silicon content of the cupola charge and add to the cost of the charge.
For a number of reasons, it would be advantageous to ignore, or at least reduce the efforts which must be made in controlling the silicon content of the initial cupola charge and during subsequent melting. Less expensive metallics, such as steel scrap, cast iron borings and steel turnings, can be used in the initial cupola charge due to the high carbon pickup and move uniform carbon curve which can be achieved with a lower silicon input. It should also be apparent that a lower silicon input facilitates a more uniform silicon curve. Another economic advantage is that the use of auxiliary silicon additives can be eliminated from the initial cupola charge. This increases the carbon recovery or pickup from the coke that is used as a fuel and for carbon pickup. Therefore, the amount of coke and air needed to melt the charge can be reduced without a loss in temperature. Reducing the amount of coke in the charge also produces an increase in the melt rate. Another advantage is that the cupola operator has greater flexibility in the type of slag run with the melt since there is less need to protect the silicon content with the slag. For example, it is normally undesirable to run a basic slag if a high silicon content is required due to the high loss of silicon caused by a basic slag. On the other hand, a basic slag is desirable if it is necessary to reduce the sulfur and phosphorous contents of the iron. If it is unnecessary to control the silicon content the cupola operator need not balance these two competing interests.
Past practice has been dictated partly by tradition and partly due to the fact that it is difficult to add significant amounts of silicon to the melt. This is due, in part, to the low specific gravity of liquid silicon (or the silicon and its carrier) with respect to the molten iron,. Hence, the liquid silicon tends to float on the surface of the molten iron. As a result, the silicon does not become mixed with the melt. Additionally, the floating silicon tends to freeze on surfaces of the holding and transfer equipment thus plugging the system.
Final adjusting of the silicon content of the melt has been attempted by additions to the transfer or pouring ladle. Heretofore, such procedures have been effective in producing only minor changes in the silicon content of the melt, measured on the order of tenths of a percent. Silicon has also been added as an inoculant to the transfer or pouring ladle, but this also produces very little change in the overall silicon content of the melt. In all heretofore known procedures, the major source of silicon has been the metallics and auxiliary additives used in the initial charge. In other words, post-melting additions of silicon have been relied upon to effect only minor changes in the silicon content of the melt. In all cases, the silicon content of the initial cupola charge has been at least equal to, and normally greater than, the desired silicon content of the melt. Thus, the accepted practice is to use a high percentage of the more expensive silicon rich metallics in the initial charge, to add auxiliary silicon bearing additives and to make special provisions to protect the silicon content during melting.